As shown in FIG. 1, for example, as a hydraulically actuated breaker, there has been known one, in which a cylinder portion 6 is constructed by inserting a piston 3 within a cylinder bore 2 of a breaker main body 1 and thus defining a first chamber 4 and a second chamber 5 at opposite sides, a chisel 8 is inserted into a chisel insertion hole 7 of the breaker main body 1, and a supply of a pressurized fluid to the first chamber 4 and the second chamber 5 is controlled to drive the piston 3 upwardly and downwardly to hammer the chisel 8.
For performing a crushing operation by mounting the above-mentioned hydraulically actuated breaker on a hydraulic shovel, as shown in FIG. 1, the breaker main body 1 is mounted on an arm 10 of the hydraulic shovel 9. A boom 13 and the arm 10 are pivoted downwardly to slightly lift a crawler 11 to apply a downward force to the breaker main body 1. In this condition, the piston 3 is driven reciprocally to hammer the chisel 8 to crush concrete 12. When the concrete 12 is crushed, the arm 10, the boom 13 and the crawler 11 are dropped together with the breaker. Then, the operator terminates the operation of the breaker, and shifts an objective position of impact by the chisel 8 by performing pivoting of the hydraulic shovel 9 or so forth in a condition where the boom 13 and the arm 10 are pivoted upwardly, to again actuate the breaker in this condition.
As set forth above, upon performing the crushing operation, if the operator cannot visually detect the fact that a crack is formed in the concrete 12 and thus it is crushed, and the operation of the breaker is continued, a penetration resistance to the chisel 8 becomes so significantly small that the chisel 8 is hammered into the crack to perform only hammering of the chisel 8 without acting the penetration resistance also acting on the chisel 8.
On the other hand, when the tip end of the chisel 8 continues to penetrate the concrete during the crushing thereof, the crawler which has been lifted, contacts with the ground surface. Subsequently, the penetration resistance does not act on the chisel 8 to perform only hammering of the chisel 8.
As set forth above, when only hammering of the chisel 8 is performed without the penetration resistance acting on the chisel, the piston 3 does not hammer the chisel 8 but hammers the breaker main body 1 (hereinafter referred to as "a lost motion"). Thus, the breaker main body 1 may be damaged. Also, the breaker is actuated wastefully to degrade an efficiency of the crushing operation.
The foregoing lost-motion will be discussed concretely. Normally, since the breaker main body 1 is pushed downwardly by a force in a direction shown by an arrow a, the chisel 8 is pushed onto the piston 3 by a penetration resistance in a direction shown by an arrow b to move a hammering position of the chisel by the piston 3 to a position c. Thus, the piston 3 hammers the chisel 8. However, when a crack is formed in the concrete 12 and thus the penetration resistance to the chisel 8 becomes significantly small, the piston 3 is lowered down to a stroke end d to hammer the breaker main body 1 without hammering the chisel 8.
As a structure for preventing the foregoing lost motion, there has been known a first construction, in which a pressurized fluid filled damping chamber for braking the piston when the piston is lowered beyond a predetermined stroke, is provided to stop the piston by the pressurized fluid filled damping chamber or to prevent a collision with the breaker main body.
On the other hand, as disclosed in Japanese Unexamined Utility Model Publication No. Showa 53-101001, there has been known a second structure, in which a hydraulically actuated switching valve actuated with a pressure of the cylinder applying a force to the breaker as a pilot pressure, is provided to make the breaker inoperative by switching the hydraulically actuated switching valve when no pressure is applied to the cylinder.
Also, as disclosed in Japanese Unexamined Utility Model Publication No. Showa 55-17791, there has been known a third structure, in which a supply opening and a discharge opening for supplying and discharging a pressurized fluid to a first chamber and from a second chamber of a cylinder portion, respectively, are communicated when the piston is lowered beyond a predetermined stroke to stop the piston.
However, in the foregoing first structure, it becomes necessary to set a damper clearance of the pressurized fluid filled damping chamber at a predetermined value and a strength of the breaker main body to be required for preventing damage of the breaker main body accompanying an elevating of pressure of the filled pressurized fluid becomes large thus making production costs high.
Furthermore, the pressurized fluid filled damping chamber is adapted to slow-down a speed of the piston and not to stop the piston. Therefore, the piston is sequentially actuated for reciprocation. If the piston is moved beyond the pressurized fluid filled damping chamber, the piston collides and hammers the breaker main body repeatedly to damage the breaker main body.
In the foregoing second structure, since the hydraulically actuated switching valve which is not used in a normal operation of the breaker, is provided and a pressure of the cylinder applying a pushing force to the breaker is detected to cause the pressure to act on the hydraulically actuated switching valve as the pilot pressure. Thus, the structure is complicated thereby making the hydraulic piping complicated.
In the foregoing third structure, since a bypass passage is formed in the cylinder portion or a cut-out is formed in the piston, machining of the cylinder portion becomes quite troublesome. Also, a leakage amount of the fluid becomes large.
Therefore, it is an object of the present invention to provide a hydraulically actuated breaker with a lost motion preventing device which can solve the foregoing problem.